Biomaterial composition and method for preparing same

ABSTRACT

A biomaterial composition for the resorption/substitution of organic supporting tissues, including 20-75 wt. % of an inorganic phase consisting of particles that include either hydroxyapatite (A) optionally mixed with tricalcium phosphate β (B), or calcium-titaniumphosphate (Ca(Ti) 4  (PO 4 ) 6 ) (C), and 80-25 wt. % of a liquid phase including an aqueous solution of a water-soluble biocompatible polymer that is cross-linkable under the effect of the pH of the medium. The composition is sterilizable, injectable and curable in biological media to form a biomaterial for replacing supporting tissues.

This application is a 371 of PCT/FR96/01196 filed Jul. 29, 1996.

FIELD OF THE INVENTION

The invention relates to an injectable composition for a biomaterial forfilling supporting organic tissues, intended to give rise to aresorption/substitution function.

BACKGROUND OF THE INVENTION

Bony substitutes based on calcium phosphate particles and a biologicaladhesive are known from the state of the art.

Thus, G. Daculsi et al. have described in Ann. Oto. Rhino. Laryngol.101:1992 the effectiveness of a calcium phosphate two-phase microporouscomposition for obliterating the mastoid cavity.

The same authors have also reported the effectiveness of a two-phasecalcium phosphate macroporous composition for surgical repair of longbones (Journal of Biomedical Materials Research, Vol. 24, 379-396) andin vertebral arthrodeses (Clinical Orthopaedics and Related Research,248, 1989, 169-175).

The usefulness of calcium phosphates in odontology has been demonstratedin a number of articles: A. Jean et al. in Cells and materials 1993; 3:193-200. "Pulpal response to calcium phosphate materials. In vivo studyof Calcium Phosphate materials in Endodontics"; B. Aliot-Licht et al. inArch. Oral Biol. 1994; 39: 481-489 "Comparative effect of calciumhydroxide and hydroxyapatite on the cellular activity of human pulpalfibroblasts. An in vivo approach."

Furthermore, JP 3 011 006 describes a cement for hard tissues includingan inorganic phase consisting of at least 60% of alpha tricalciumphosphate and of hydroxy-apatite and/or a calcium monophosphate, and aliquid phase including carboxymethyl cellulose.

However, such a composition exhibits the disadvantage, due to theexcessive solubility of α tricalcium phosphate, of not beingsufficiently stable to permit a process of resorption/substitution ofthe hard tissue. Furthermore, such a composition is liable to give riseto detrimental inflammatory processes. Furthermore, this mixtureconstitutes a calcium ionomer which is not suitable for injection aftera few minutes, due to hardening of the mixture as soon as it is made up.This combination exhibits a double instability, a contraction in volumewith release of water after several days and, above all, a drop inviscosity after sterilization of the mixture in the autoclave. It doesnot make it possible to produce a material which is "ready for use",sterile and injectable.

OBJECT OF THE INVENTION

The object of the present invention is to provide a biomaterialcomposition filled with mineral phase, capable of being reinhabited,injectable and curing in situ in the implantation site.

In particular, this composition must exhibit the following properties:

it must be sterilizable;

it must not be toxic in vivo;

it must have a high mineral filling inducing a mineralization frontand/or a tissue cicatrization;

it must include a dispersing agent acting as a vector which carries themineral filler into the operating site and which then holds it at thissite until the tissue cicatrization, while acting as a matrix forcomposite material;

it must be capable of being introduced into a biological medium,especially by injection with a syringe or an apparatus of "lentula" typeemployed in dental surgery (a so-called lentulable composition), in thefluid or pasty state, before curing in contact with the bufferedbiological fluids;

it must be stable in order to be capable of being stored for arelatively long time before use;

it must be easy to use.

SUMMARY OF THE INVENTION

This objective has been attained by the present invention, thesubject-matter of which is a composition for a biomaterial forresorption/substitution of supporting organic tissues, comprising:

20 to 75% by weight of an inorganic phase consisting of particlesincluding either hydroxyapatite (A), optionally mixed with β tricalciumphosphate (B), or calcium titanium phosphate (Ca(Ti)₄ (PO₄ )₆) (C), and

80 to 25% by weight of a liquid phase including an aqueous solution of awater-soluble, biocompatible polymer self-crosslinkable under the effectof the pH of the medium.

The inorganic phase based on particles of calcium phosphate(s) providesthe mineral filler needed for the mineralization front.

The calcium titanium phosphate (CTP) of formula Ca(Ti)₄ (PO₄)₆ ispreferably of the "Nasicon-like" calcium metal phosphate type.

The inorganic particles advantageously include a mixture ofhydroxyapatite (A) and of β tricalcium phosphate (B), these mixturesbeing commonly referred to by the acronym BCP (biphasic calciumphosphate). The mixture preferably includes the compounds A and B in anA/B weight ratio of between 80/20 and 30/70, in particular ofapproximately 60/40.

This latter type of filler consists of a high-temperature frit, groundand classified into powder or into granules whose particles have adiameter of 80 μm to 200 μm when the composition is prepared. Othertypes of synthesis of fillers or of other particle sizes may, however,be selected, depending on the tissues and the indications. The choice ofthe particle diameter may be guided in particular by the desiredresorption kinetics and rheology. Particles of diameter smaller than 80μm generally have fairly rapid resorption kinetics. They find theirapplication especially in orthopaedic surgery. For endodonticapplications in dental surgery, particles of diameter greater than 200μm should be avoided because they pose problems on injection, due to therheology of the composition.

Other mineral fillers may also be employed, such as the "Nasicon-like"ceramics (CZP), phosphocalcium ceramics or bioglasses.

The liquid phase is used, on the one hand, as a dispersing agent for theparticulate mineral filler, capable of mixing intimately with thelatter, it being possible for the final composition to be in a liquid ormore or less pasty form. On the other hand, it is intended to form amatrix of composite material incorporating the mineral filler.

For this purpose the liquid phase includes a polymer which isself-crosslinkable in aqueous solution.

Bearing in mind the application to a biomaterial, the polymer must bebiocompatible, that is to say that it must not be toxic and must notresult in a rejection reaction when it is implanted into the organism.Furthermore, its crosslinking must not release any toxic by-product.

In addition, the polymer must be stable in the uncrosslinked state in agiven medium and self-crosslinkable merely by being brought into contactwith the biological medium into which it is implanted. The polymerswhich best meet this criterion are those which can crosslink under theeffect of the pH of the buffered biological fluids. These polymers arepreferably soluble in the uncrosslinked state in a relatively basicaqueous phase, and crosslink under the effect of a drop in pH.

It is particularly advantageous that the polymer should crosslinkcovalently in the implantation medium, to produce a strong crosslinkedbiomaterial.

Consequently, preference is given to polymers which, at the pH of themedium, are capable of undergoing a chemical reaction resulting in theformation of intermolecular and possibly intramolecular covalent bonds.

Such polymers may be advantageously selected from polymers having sidegroups containing silicon-based reactive functional groups, such asalkali metal or ammonium silanolate groups or organosilicon groups whichcan be hydrolysed in the biological medium to form silanolates withoutreleasing any toxic product as hydrolysis by-product.

A suitable silanolate side group may be, for example, a group of formula

    --CH.sub.2 --CHOH--CH.sub.2 O--(CH.sub.2).sub.3 --Si--(ONa).sub.3.

To provide a degree of crosslinking that is suitable for the applicationto a curing biomaterial, it is desirable that the silicon-carrying sidegroups of silanolate or silanolate precursor type should represent from1 to 5% of the total dry weight of the said polymer.

The base polymer to which the side groups are bonded may be of variousnatures, provided that it is biocompatible.

It may be advantageously selected from cellulose and polymers derivedfrom cellulose, whose compatibility is well known and applied ingalenics to the delay matrices for medication. Nonionic cellulose ethersmay be mentioned in particular, for example hydroxyethyl cellulose,hydroxyethyl methyl cellulose or hydroxypropyl methyl cellulose.

Thus, a polymer which is self-crosslinkable by covalent bonds andderived from cellulose may be obtained by etherification of cellulose orof a derivative thereof by reaction with a compound of formula (1)

    XSi(OZ).sub.3                                              (1)

where X denotes a halogen atom or a hydrocarbon group containing anepoxy functional group, especially C₂₋₂₀, and Z is selected from ahydrogen atom, an alkali metal and an alkyl group, especially C₁₋₅.

The compound of formula (1) may be, for example,(3-glycidoxypropyl)trimethoxysilane ##STR1##

The synthesis of polymers derived from hydroxyethyl cellulose (HEC) andfrom (3-glycidoxypropyl)trimethoxysilane is described by Arjun C. Sauand Thomas G. Majewicz in Cellulose Ethers; Self-cross-linking mixedether silyl derivatives, ACS Symp. Ser. 1992, Vol 476, 265-72.

In a basic medium the organosilicon compound is grafted onto the HECwith opening of the epoxide, and the methoxysilane groups arehydrolysed, to produce a polymer corresponding to the simplifiedformula: ##STR2## This polymer is stable in aqueous solution at a pHhigher than or equal to approximately 10.5. Acidification of thesolution results in a gradual increase in viscosity and the formation ofa hydrogel. This physical phenomenon corresponds to the crosslinking ofthe polymer by (i) conversion of the silanolate groups into silanolgroups: ##STR3## and then formation of a threedimensional network by(ii) condensation of a silanol with another silanol ##STR4## and/or(iii) condensation of a silanol with a hydroxyl group of the rings ofthe cellulose ethers or of the substituents ##STR5##

This crosslinking of covalent type, produced by a drop in the pH of theaqueous solution of the polymer, is reversible and the hydrogelredissolves when the pH of the medium is increased.

Such a polymer can be in the dissolved state in a composition accordingto the invention containing BCP, by virtue of the basicity of thesecalcium phosphate mixtures. The composition may also contain a base ofany type in order to provide the alkalinity needed for dissolving thepolymer.

In the composition according to the invention an auxiliary crosslinkingof ionic type can also take place at the silanolate groups bridged viathe divalent Ca²⁺ cations.

The functionalization by self-crosslinking silanolate groups can beapplied to any other type of water-soluble and biocompatible polymerexhibiting a suitable reactivity. Other polysaccharides may bementioned, especially guar, starch and their etherified derivatives.

Because of its two components, the composition according to theinvention forms a system which has a high mineral filling, and isself-crosslinking in contact with buffered biological media, without anyadjuvant or catalytic system for bridging, the crosslinking of thepolymer being triggered by the change in the pH of the composition.

The crosslinked material obtained will be more or less dense, dependingon the polymer content and the quantity of crosslinkable functionalgroups in the said polymer.

The liquid phase also determines behaviour with regard to therheological properties and therefore to the viscoelasticity of theuncrosslinked final composition intended to be implanted in thebiological medium.

To this end, the concentration of self-crosslinkable polymer isadvantageously between 1 and 5% by weight, preferably approximately 2%by weight relative to the weight of the liquid phase.

In an alternative form the liquid phase of the composition mayadditionally include a noncrosslinkable biocompatible polymer, and thismakes it possible independently to determine the rheological propertiesof the composition before injection and the degree of hardening of thematerial implanted in the medium. Any water-soluble biocompatiblepolymer may be employed for this purpose, especially polysaccharides orderivatives such as cellulose and cellulose ethers. The relativeproportions of the polymers will be adjusted conventionally as afunction of the desired properties.

The composition according to the invention is obtained by mixing theconstituents of the inorganic phase and of the liquid phase.

The β tricalcium phosphate and hydroxyapatite or CTP granules or powderof the inorganic phase may be obtained as described by Daculsi et al.(Rev. Chir. Orthop., 1989, 75 (2): 65-71).

A major advantage of the composition according to the invention is thatit is sterilizable, before or after the mixing of the two phases.

The invention therefore also has as its subject a composition asdescribed above and sterile.

Conventional sterilization with ethylene oxide is not possible in thecase of such a "ready-for-use" material. The components of the mixturemust, in fact, be sterilized in their dry form, and this demandshandling by the surgeon before the injection. This handling is difficultand not reproducible.

According to the invention the preparation of a sterile composition iscarried out by dissolving the polymer constituent of the liquid phase inwater to a viscosity determined as a function of the desired finalviscosity. The solution obtained is next mixed with the inorganic phaseand the resulting composition is introduced into packaging bottles whichare sealed and sterilized in the autoclave at 121° C. for 20 minutes.

Depending on the requirements, the autoclave sterilization of themixture may also be performed at a temperature of 130° C. for 30 minutesif the dissolving potential of the filler is not too great.

The initial viscosity of the composition (polymer concentration) must beadapted to obtain the desired viscosity after sterilization, that is tosay the polymer concentration as defined above.

If the dissolving potential of the phosphocalcium filler does not allowthe mixture to be sterilized at 121 or 130° C. because of rheologicalproblems at this temperature, the aqueous liquid phase is sterilized inthe autoclave separately from the filler in one of the above conditions,and then the mixing of both sterile parts is performed in a sterilewhite room.

The invention also relates to a kit for the preparation of a compositionfor a biomaterial, including, on the one hand, a sterile inorganic phaseand, on the other hand, a sterile liquid phase, as described above,which are intended to be mixed extemporaneously under sterile atmospherebefore implanting.

The composition according to the invention can be stored in itsready-for-use mixed or ready-for-mixing separate form withoutappreciable loss in quality.

The composition according to the invention may be employed as materialfor bony filling of supporting organic body tissues, this material beingintended to give rise to a resorption/substitution function. It may inparticular be employed as a filling material in combination with jointimplants or prostheses or for any surgical application and indicationrequiring some "toughness" or mechanical properties.

Another subject of the invention is therefore a process for thetreatment of the human or animal body, including the administration byinjection of a composition according to the invention at a site which isnormally occupied by a supporting organic tissue, to give rise to afunction of resorption/substitution of this tissue.

An example of application is dental surgery. The injection may becarried out with the aid of a system comprising a sterilizable syringeand end fittings provided with plungers for single use, for example thesystem marketed by Hawe Neos Dental, including a syringe sterilized inthe autoclave (Ref. No. 440, Hawe-Centrix C-R®, Mark III syringe) andend fittings (Ref. No. 445).

Another example of application of the composition is orthopaedicsurgery, in which it can be injected especially for treating theproblems of instability of the lumbar rachis, or as a filling material(sealing agent) used in combination with a joint prosthesis, for examplea hip prosthesis. The composition may be injected percutaneously.

BRIEF DESCRIPTION OF THE DRAWING

The curve in the single FIGURE shows the change in the torque moment ofa viscometer spring measuring the change in the shear stress as afunction of time.

DETAILED DESCRIPTION OF THE INVENTION

An example of composition according to the invention will be givenbelow, the properties of which are illustrated in particular in thesingle FIGURE which shows the hardening curve of the composition as afunction of time in a medium buffered at pH 7 at 25° C.

EXAMPLE OF COMPOSITION

A self-crosslinkable polymer is prepared from a nonionic cellulose etherin the following manner:

An inert nitrogen atmosphere is established in a reactor with magneticstirring, fitted with a condenser, and 1 l of propanol, 20 g ofhydroxyethyl cellulose of high molecular weight (which is insoluble inthe propanol) and 1 g of sodium hydroxide are introduced. After onehour's stirring 0.4 g of 3-glycidoxypropyltrimethoxysilane are added tothe suspension of cellulose polymer (that is, 2% by weight relative tothe weight of polymer).

The mixture is heated for 1 h to 80° C. and then for 2 h under reflux atapproximately 110° C.

Stirring is continued overnight without heating and the polymer is thenfiltered off and then washed with propanol, with acetic acid, withpropanol again and finally with acetone.

The silanized polymer is dissolved in a decimolar aqueous solution ofsodium hydroxide (pH=13) for 3 days, in a proportion of 2% by weight ofdry polymer in the aqueous solvent.

30 g of this liquid phase are mixed with 20 g of a mixture of 40% of βtricalcium phosphate and 60% of hydroxyapatite.

This mixture is placed in filter paper which is in the form of a tubeclosed at one end, usually employed for an extractor.

The filter filled with the mixture is placed in a vessel containing abuffer solution of 9% NaCl, of pH=7, at a temperature of 25° C.,modelling a biological medium. The filter allows ion diffusion betweenthe mixture and the buffer solution.

The curing of the composition due to the drop in pH is followed bymeasuring the change in the viscosity of the mixture in the course oftime, by means of a Brookfield viscometer fitted with a needle No. 29for small samples, with a slow shear rate of 1 revolution per minute toavoid the rupture of the gel being formed.

The curve in the single FIGURE shows the change in the torque moment ofthe viscometer spring (measuring the change in the shear stress) as afunction of time.

It shows that the mixture cures spontaneously at pH 7 in a progressivemanner. With the initial viscosity being 141 Pa s, this reaches 1000 Pas after 41 hours, whereas the pH of the buffer solution has not changed.

The composition prepared in this example can be sterilized in theautoclave at 121° C. for 20 minutes.

TOXICITY TEST

In a paper in the Journal of Applied Biomaterials Vol. 3, 197-206(1992), K. P. Andriano et al. have shown the noncytotoxicity of silaneson direct contact of silane-treated mineral fibres with L 929 mousefibroblast cells.

It is also verified that hydroxyethyl cellulose modified with theepoxysilane is not cytotoxic. Using L 929 cells, direct deposition ofthe cells on the polymer which is silane-treated and dried into filmshows 97 to 99% of living cells after 24 hours in direct contact.

In an indirect (transwell) test, in which the cells are deposited on alayer of gel and the polymer is placed on the other side of the gelledlayer, 97 to 98% of living cells are also found again after 24 hours,which shows that the products of diffusion through the gel are nottoxic.

The silane-treated polymer is not cytotoxic according to Afnor standardNF S 90 702.

We claim:
 1. A composition for a biomaterial for resorption/substitutionof supporting organic tissues, comprising:20 to 75% by weight of aninorganic phase consisting of particles including either hydroxyapatite(A), optionally mixed with β tricalcium phosphate (B), or calciumtitanium phosphate (Ca(Ti)₄ (PO₄)₆) (C), and 80 to 25% by weight of aliquid phase including an aqueous solution of a water-soluble,biocompatible polymer self-crosslinkable under the effect of the pH ofthe medium.
 2. The composition according to claim 1, wherein saidinorganic phase comprises a mixture of hydroxyapatite (A) and of βtricalcium phosphate (B) in an A/B weight ratio of 80/20 to 30/70. 3.The composition according to claim 1, wherein said inorganic phasecomprises a mixture of hydroxyapatite (A) and of β tricalcium phosphate(B) in an A/B weight ratio of approximately 60/40.
 4. The compositionaccording to claim 1, wherein said inorganic phase consists of a powderwhose particle size when the composition is prepared is between 80 and200 μm in diameter.
 5. The composition according to claim 1, whereinsaid polymer self-crosslinks forming covalent bonds under the effect ofthe pH.
 6. The composition according to claim 1, wherein said polymercontains side groups of alkali metal or ammonium silanolate orsilanolate precursor.
 7. The composition according to claim 1, whereinsaid polymer contains side groups of alkali metal or ammonium silanolateor silanolate precursor, the side groups representing from 1 to 5% ofthe total dry weight of the said polymer.
 8. The composition accordingto claim 1, wherein said polymer is derived from a polymer selected fromthe group consisting of cellulose, a nonionic cellulose ether, guar andstarch.
 9. The composition according to claim 1, wherein said polymer isderived from a polymer selected from the group consisting of cellulose,a nonionic cellulose ether, guar and starch, and said polymer resultsfrom the etherification of cellulose or of a derivative thereof with acompound of formula (1) XSi(OZ)₃ where X denotes a halogen atom or ahydrocarbon group containing an epoxy functional group, and Z isselected from the group consisting of a hydrogen atom, an alkali metaland an alkyl group.
 10. The composition according to claim 1, whereinthe concentration of said self-crosslinkable polymer in the liquid phaseis from 1 to 5% by weight.
 11. The composition according to claim 1,which is sterile.
 12. A method for the preparation of a composition fora biomaterial for resorption/substitution of supporting organic tissues,comprising:20 to 75% by weight of an inorganic phase consisting ofparticles including either hydroxyapatite (A), optionally mixed with βtricalcium phosphate (B), or calcium titanium phosphate (Ca(Ti)₄ (PO₄)₆)(C), and 80 to 25% by weight of a liquid phase including an aqueoussolution of a water-soluble, biocompatible polymer self-crosslinkableunder the effect of the pH of the medium, comprising the followingsteps:preparing a liquid phase by dissolving said polymer in an aqueoussolvent, mixing said inorganic phase with said liquid phase, andsterilizing the mixture thus obtained.
 13. The method according to claim12, wherein the sterilization is performed in an autoclave at atemperature of 121 or 130° C.
 14. A kit for the preparation of acomposition according to claim 1, which is sterilized, comprising:asterile inorganic phase consisting of particles including eitherhydroxyapatite (A), optionally mixed with β tricalcium phosphate (B), orcalcium titanium phosphate (Ca(Ti)₄ (PO₄)₆) (C), and a sterile liquidphase comprising an aqueous solution of a water-soluble, biocompatiblepolymer self-crosslinkable under the effect of the pH of the medium,which are intended to be mixed extemporaneously under sterile atmospherebefore implanting of the composition.